@Sir Gregor,
Update your Unity Player. It's a really small download and it's a quick install (a few seconds). Hope this helps.
https://unity3d.com/webplayer
For the swarm waves, just max your Poison Mastery skill and only use traps with poison gems in them. Make sure you space them out so that if the traps miss any of the monsters, you have a couple backup traps closer to your tower. You shouldn't need more than 3 or 4 traps for most levels.
If you add a tower close to all the enemy entrances (and don't worry about upgrading them), you can make sure that the ninjas get shot and go invisible when they enter the map. By the time they get down closer to your gems, they are visible again and your more powerful towers can fire upon them.
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